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Forum Index : Electronics : Turning GTI off when batteries are charged
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| Godoh Guru  Joined: 26/09/2020 Location: AustraliaPosts: 457 |
I am toying with getting a GTI inverter to use to charge the car. On sunny days I can use my current 15 amp EV charger fine, but would like to be able to run a bigger charger while the sun is out in clear skies. So I am thinking of getting a second hand GTI inverter and using it to run the car charger. I have about 10kw of panels now, so was figuring that when the batteries are nearly full I could run a biggish GTI to charge the car. The only thing I am not sure about is how to stop the batteries from overcharging. I know that the GTI needs the battery inverter to act as the grid, but how do I throttle back the battery inverter when the batteries are full and still keep the GTI going? Also wondering if my Aliexpress board based inverters ( 8010) will be fine for this situation. The main one I will use is about 4kw output. But I have never seen the batteries take more that 2kw at 24 volts. So there should be plenty of power left for the car on sunny days. Any suggestions welcome. Pete |
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Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Pete, the GTI throttling has been discussed on the forum. Basically there are two ways: 1. AC on/off controlled by a voltage (battery) sensing relay. I use this method, it is simple and very reliable but the batteries must be able to accept the full charge the GTI can supply if they are a bit low. 2. Use a PWM controller to switch IGBT's to control the DC input voltage to the GTI. I think Madness uses this method - search for his posts if you are interested. Myself does not like to switch high voltage DC, trouble if something shorts out. Method 2 gives gradual charging control while method 1 is all or nothing. I find the best way with method 1 is using it for bulk charging (set the voltage control Rly appropriately) and also use MPPT charging to top up & maintain the batteries charged. No problem using MPPT and GTI charging at the same time - has worked for me for a few years now. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Thanks Murphy, I did a search here but did not have the right term to find what I wanted. The battery voltage sensing relay sounds good, but now i am wondering if it will work for me. I was thinking of getting a 7 or 8kw GTI so I could charge the car at 7kw. But there is no way the batteries will take that much charge. I have looked at Hybrid inverters but as my system is 24 volt they only seem to go up to 4200 watts, which is not much more than my current home made inverter can do. They make bigger hybrid inverters but they are all for 48 volt or higher batteries. Seems that it is back in the hard basket. Thanks again for the reply Pete |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Pete, what happens is: *only* if there is no load, the batteries get the full power from the GTI as charge. Any AC load gets powered directly from your mini grid (via the GTI) and only What the batteries need goes to your inverter charging the batteries. You'll need something to pull the plug (switch off the GTI) when the car is charged. A battery voltage sensing relay could do that. 7 or 8KW means a lot of solar and a rather big GTI. Too bad you are still sticking to 24V  , it's going to bite you in the long run with your extra power requirements. |
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| analog8484 Senior Member  Joined: 11/11/2021 Location: United StatesPosts: 108 |
The common way to curtail GTI output power in commercial inverters is called freq-watt control where the AC output frequency is increased (max ~0.5 Hz per sec to avoid problems) up to 5Hz above the nominal line frequency. GTI's are typically have default settings output power at 100% of capacity at the nominal line frequency and start curtailment (e.g. 90% of capacity) at ~0.2Hz above the line frequency and completely off by 2Hz above the nominal line frequency. Most modern GTI's have configurable settings so the frequency setpoints can be adjusted. Conceptually freq-watt control should be doable with a nanoverter with some mods to monitor the battery bank SOC and start increasing frequency at near-full SOC (e.g. 95%) until the inverter/charger see no charging current. Battery bank voltage could be used instead of SOC for simpler integration but it's known to be less reliable for lithium based batteries. Edited 2024-06-29 01:01 by analog8484 |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 512 |
control a Gti Here is the awesome Oztules explanation... helped me out heaps... but as has been said, there are numerous threads on this if you search... I think it works !! |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Thankyou for the answers. I am sticking to 24 volt at the moment because of the cost of changing over. It would add up to a lot of money for new batteries, new charge controllers, new inverters ( mine are all 24 volt) and new down converters for my 12 volt stuff. It is just beyond what I want to spend at the moment. Thanks for that link Noneya, I do remember Oz doing that article. I wish he was still around, he was a pretty awesome character. I will read through his article again. Thanks again for the help folks Pete |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Hi Murphy, your explanation helps. Am I right in thinking that the GTI and the Battery inverter can share the load? I am thinking I can use the voltage sensing to sense when the batteries are full, then use a relay to switch the GTI off when the car is full or when the batteries are full. If the GTI and the Battery inverter can work together and share the load then I only need a 3 or 4kw GTI to do the job. Then the Battery inverter and GTI together can be used to charge the car. Or the GTI can then be used when the car is charged to help charge the batteries or just be turned off. Pete |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Pete, it's no different than a lot of GTI's feeding power into a street grid. Each contributes what it can. The problem is when your mini grid needs no more power (batteries full in your case). As the GTI power contribution (in my set up) is not regulated it needs to be turned off to avoid cooking the batteries. The easiest way to do that is to disconnect the GTI from your battery inverter (it's what I do). You need to sense the battery voltage (what kind of batteries do you have?) to control an AC switching relay to disconnect the GTI when the batteries are full. I switch both, active & neutral. Choose your voltage sensing relay and AC switching relays coil voltage to suit your battery bank voltage. This will automatically re connect when the batteries reach your lower set level (relay hysteresis setting). If the GTI has enough solar input by then, then it also goes through its grid re connecting procedure. |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Well, I too started up with a 24V system Pete. If you have more than two 12V batteries it would not be difficult to re connect to 48V. Expense? a few links. If you had built your own inverter (like many here have), it's just a matter to add turns to the primary, you can use thinner wire for the same power. Home built inverters work better than that over rated Chinese stuff  , you can repair it too (hardly required with the new designs here, unless something silly is done to it.Charge controllers? I don't know what you have but many can sense the battery voltage and adjust automatically. Perhaps a good excuse to start building that MPPT I sent you some PCB's for .!2V down converters? If you have a mains inverter then a 12V power supply should cater for that. Just some ideas, look at it as a longer time project so any expense can be spread out. |
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| analog8484 Senior Member Joined: 11/11/2021 Location: United StatesPosts: 108 |
That's a key reason for AC coupling with GTI. It avoids/reduces the loss of DC(PV panel output) -> AC (GTI output) -> DC(battery) -> AC(battery inverter output) for power loads from GTIs. Edited 2024-06-30 04:02 by analog8484 |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Thanks Analog, that sounds good, I have my eye on a 5kw GTI that is second hand, I will hook that up to 3.5kw of panels and see how it goes. Murphy at the moment our batteries are 12 x 2 volt BAE VRLA cells connected in series. It would cost about $7k to add another bank. All up I have 4 inverters, all 24 volt, I would have to give them away or sell them to go 24 volt as two are commercial Latronics ones and two are home made aliexpress 8010 boards. I have 8 MPPT controllers, but some of them will become spares, if the GTI unit works out as you have pointed out. So that will make my setup simpler. I will let you know how it goes. I will also keep my options re 48 volt open, it will be a lot of work buy may be well worth it in the long run. As for making the MPPT controllers that you sent me the boards for. I have most of the components but not the Hall effect unit yet. There was some reference from you about another component that goes on the bottom of the board too. But I am not up to building that yet. Murphy, is the anti islanding fast enough on the GTI when you switch the battery inverter off that it doesn't send spikes through the battery inverter? Thanks Pete |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Pete, I do not switch the battery inverter off, I disconnect the GTI *AC* from the mini grid with the battery inverter. The GTI then does its thing with "no grid connection" and eventually turns itself off. Re connection takes the usual checking time once there is enough solar to wake it up. I never looked for any spikes, the disconnect is as fast as the relay contacts open. My inverters are all home made, no idea how your Chinese inverters behave. You need AC rated relays with contacts that easily handle full GTI AC loads . |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Thanks Mr Murphy. I found a voltage controlled relay in my box of bits. So will use that to control a contactor or relay to do the same. I have just read pages of Madness's controller that he made in the vein of Oztules controller. It seems to work but sounds like it is out of my league. Your way sounds more up my alley. Simple and foolproof. Thanks again for your help. I am going to get a 5kw Zeversolar inverter on Thursday. Will report back on how it all goes. Pete |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Just an update. It is totally foggy here today, So i got to work on reconfiguring my panels and connecting the GTI inverter up. The good news is that my home made inverter works fine as the grid. It is charging the house batteries now from the GTI inverter. I reconnected my 3600 watt panel rack to have 2 banks of 4 panels in series, that gives me a panel voltage of around 180 volts DC open circuit. The inverter has two MPPT controllers so I decided to use them both. I am pretty happy with the result so far. At the moment I am using the Victron BMV that I have to disconnect the GTI from the Battery inverter when the batteries are fully charged. The BMV has a relay in it that can be set to turn on or off at set points. So when the batteries are charged the BMV relay will de energise a 240 volt 30 amp relay to simulate the grid turning off. Time will tell but so far smiles thankyou to the folks on this site for the help and information Pete |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Good news Pete, you are now a member of the mini grid club .I do hope that BMV relay turns on and off at different voltages. If not each slight dip of the battery voltage will trip the GTI on again. Perhaps you already have set your MPPT charger to supply small daytime loads without dropping the battery voltage at all. The GTI will do your heavy charging then. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Hi Mr Murphy, yes the BMV has upper and lower voltage settings that can be used. It can also be used to turn stuff off an on depending on the state of charge of the battery too. I will monitor it and see which suits best Of course there will be a manual switch too so that I can run those inverters to power the shed as well. I am in the process now of thinking through how to shut the Battery inverter down when the GTI drops its output too much. My thinking is that when they are load sharing charging the car that the battery inverter will be overloaded if the grid inverter drops its output too much. So I am thinking of using a current controlled switch on the GTI output that will drop the battery inverter out when the GTI output goes too low. Most of the time I will have the units setup to mostly use the GTI output to run the car charger, but they will be sharing the load on sunny days. I have some current relays coming so will get my head around the settings when they come Cheers Pete |
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| analog8484 Senior Member Joined: 11/11/2021 Location: United StatesPosts: 108 |
If you want to avoid "wasting" solar energy you can also use the relay to turn on/off opportunity loads (e.g. hot water heater,EV charging, etc.) to use excess solar power instead of simply shutting off the GTI. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 457 |
Thanks analog. In winter we are using a wood heater to heat the house and hot water, so we don't have any other loads to use excess power on. Today I charged the car, I was putting 15 amps into the car and the GTI was putting out about 2300 watts with the Battery inverter supplying the rest. Our panels were easily keeping up as it was a sunny day. The only thing I found was that when I changed the load on the battery inverter the grid inverter would trip out as if it had no grid. Maybe the small dip in the battery inverter output voltage was enough for the GTI to disconnect. Once I had changed the charging current and got the car charging again, the GTI then found the fake grid from the battery inverter and did most of the heavy lifting. I have the GTI connected to 8x 435 watt panels, but being winter the solar power is not as high as I would expect in summer. Still very happy with how the system is working. Pete |
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| analog8484 Senior Member Joined: 11/11/2021 Location: United StatesPosts: 108 |
Yeah, that's most likely due to the GTI anti-islanding mechanisms that intentionally disturb the island grid by shifting frequency and/or phase of its output current especially when it detects significant rapid changes (e.g. large loads turning on/off). For a typical off-grid inverter, unless it has much higher power capacity (>2x) than the GTI then the voltage waveform can become highly distorted by the intentional disturbances causing the GTI to trip out. Some modern GTI's allow turning off anti-islanding so you may want to check to see if your GTI support the option. It allows for much smoother operation under large load changes. |
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